Ultrasound scanning in early pregnancy
Assessment of early pregnancy is dealt with elsewhere, as the complication encountered most often relates to threatened miscarriage.
The 11 – 13 week first trimester scan
As we approach the end of the first trimester the developing embryo has now become a fetus, attaining the form and shape of a small human (12wk1). At this stage of pregnancy we can now discover much about a pregnancy:
- Confirm viability – the majority of miscarriages will have taken place by the end of the first trimester. If the fetus appears healthy at this stage of the pregnancy, the risk of subsequent miscarriage is very low (~1%). In a small number of pregnancies a missed miscarriage may be found (the pregnancy is not viable but the placenta or afterbirth continues to be active, having missed the fact.
- Confirm gestational age. When we count the number of weeks pregnant from the first day of the last period, we assume that conception took place exactly two weeks later. This is often not the case. The ultrasound measurement provides a more accurate estimate of gestational age, though we usually don’t redate unless there is more than a week in the difference between the menstrual and ultrasound dates.
- Confirm number of fetuses – This is the best time to check for twins and for the chorionicity of the twins.
- Identify abnormalities – Many abnormalities can be seen at this stage, particularly the more severe abnormalities.
- Screening for Chromosomal anomalies – Our genes are carried on our chromosomes, which are in every cell in our body. The natural process is not perfect, and sometimes there are too many or too few chromosomes, leading to problems such as mental retardation. The only way of being certain whether a fetus has a chromosomal anomaly or not is to perform an invasive procedure, such as amniocentesis, placental biopsy (CVS) or fetal cord blood sampling (see prenatal diagnosis). These tests carry a 1% risk of miscarriage, so many women only wish to consider an invasive test if they are high risk. Screening tests, looking at the fluid at the back of the neck of the fetus (translucent neck – nuchal translucency) (12wk2) and biochemistry assay of placental hormones. The most popular of these tests is the combined Nuchal/biochemistry test, performed around 11 – 13 weeks gestation.
The mid-pregnancy ultrasound assessment (16 wk)
At this stage of pregnancy we can now discover much about a pregnancy:
- Confirm gestational age. If dates, viability and number of pregnacies have not been confirmed by the 11 – 13 weeks scan (see back), they can be permed at this time. Identify abnormalities – Many abnormalities which cannot be seen can be seen at this stage, particularly the more severe abnormalities (holo 4)
- Identify abnormalities – Many abnormalities cannot be seen at 11 – 13 weeks, so they can be identified at about 20 weeks gestation (see prenatal diagnosis).
- Screening for Chromosomal anomalies – Our genes are carried on our chromosomes, which are in every cell in our body. The natural process is not perfect, and sometimes there are too many or too few chromosomes, leading to problems such as mental retardation. The only way of being certain whether a fetus has a chromosomal anomaly or not is to perform an invasive procedure, such as amniocentesis, placental biopsy (CVS) or fetal cord blood sampling.
- These tests carry a 1% risk of miscarriage, so many women only wish to consider an invasive test if they are high risk. If the patient has not had nuchal translucency or biochemical screening (see above) markers or possible signs can be sought on the 20-week scan. If earlier screening has taken place, many of the original markers do not substantially alter the risk. If there is more than one marker a careful review by a Fetal Medicine Specialist is recommended.
- Screening for pre-eclampsia and IUGR – These conditions carry great risks for the mother and fetus later in the pregnancy. These tests carry a 1% risk. Using ultrasound blood flow and biochemistry tests, it is possible to identify women who are at increased risk of these conditions.
- Screening for preterm labour – Preterm labour and delivery is one of the biggest causes of death and handicap in the newborn (see common obstetric problems). A weak cervix or neck of the womb is one cause of preterm labour. Ultrasound measurement of the cervix between 20 and 26 weeks can be helpful in spotting some of the women that will deliver their baby early.
Uterine artery Doppler is discussed in greater detail later in this chapter.
Scanning in the second half of pregnancy
In the U.K. the majority of women will have had an ultrasound scan to confirm viability, number of fetuses and to check for any obvious anomalies (first trimester scan). The purpose of scanning in the second half of pregnancy is to help diagnose and manage pregnancies at risk of obstetric complications. We use ultrasound to:
- Measure the fetus and assess growth.
- Assess fetal wellbeing by looking at fetal movement and liquor (amniotic fluid) volume (3trim1)
- Check the fetal blood flow, to check for reduced flow or redistribution of flow (associated with a reduced oxygen supply to the fetus).
Where there is normal growth, activity and liquor, there is generally no cause for concern. The assessment is less useful for women who go beyond their due date, as the risk of unexpected serious events is greater in this group of women.
If there is a reduction in growth, poor movement or reduced liquor, or changes in the blood flow, there is cause for concern. Further scans or early delivery (often by Caesarean Section) is required.
The 11 – 13 week first trimester scan – more detailed information.
Throughout the nineties we have sought to identify the best use of ultrasound in the first half of pregnancy, differentiating between the limitations of ultrasound and its users, constantly trying to provide the most accurate data on the capability of this investigation. Ultrasound manufacturers have taken advantage of revolutionary developments in computer technology to produce machines and transducers with ever-greater resolution, particularly at higher frequencies.
These machines, whether transabdominal or transvaginal, have made routine investigation in early pregnancy a reality. Early pregnancy dating, confirmation of viability, diagnosis of multiple pregnancy and abnormality, can now be attained with a 10-14 week scan. It is therefore apparent that much of what is currently achieved at the mid-pregnancy assessment can be fulfilled by the 12-14 week scan. Although earlier diagnosis includes pregnancies that would have been lost through the process of natural wastage, it allows women to make decisions about their pregnancy at a much earlier stage than before. Invasive prenatal diagnostic testing can also be offered at this stage, accepting the potential dangers of early chorionic villous sampling and early amniocentesis.
Nuchal translucency is a transient finding in early pregnancy, observed at a time when dramatic changes are occurring in the uteroplacental and fetal circulations. The transient nature probably reflects the completion of these changes in uteroplacental and fetal haemodynamics. Within a few weeks the fetal circulation adapts and the fluid typically disappears. The confirmation that increased fetal nuchal translucency is associated with an increased risk of chromosomal abnormality, as well as structural; particularly cardiac and genetic, syndromes, raises the possibility of adapting (or applying) the early scan to meet the needs of the original mid-pregnancy assessment. Two important issues then arise; the true ability of the early pregnancy scan to achieve these objectives, and the future role, if any, of the second trimester scan.
Lively discussions take place at every scientific meeting regarding the true capability of the early pregnancy assessment. It is now generally agreed that the estimation of gestational age is best done using ultrasound, early in the pregnancy. The detection of anomalies, whether using transvaginal or transabdominal probes, continues to improve, again through the use of better training and technology. We are still learning about the best means of exploiting the relationship between increased nuchal translucency, placental biochemistry and chromosomal and fetal abnormality.
The mid-pregnancy ultrasound assessment – more detailed information
The timing of the original pregnancy scan arose because of several factors. The initial quality of early ultrasound machines made diagnosis of anomalies difficult in the early stages of pregnancy, although reasonably accurate dating was possible before the inception of real-time B-mode scanning. With the introduction of amniocentesis for advanced maternal age, an ultrasound scan between 16 and 19 weeks gestation became more valuable, as not only could dates be confirmed, but the optimal site for the introduction of the amniocentesis needle could also be identified. Biochemical screening, initially for neural tube defects, and later for Down’s syndrome, rely on ultrasound dates to provide an accurate gestational age.
It is a sign of the pace of change in the use of ultrasound in pregnancy, that we are discussing the changing shape of mid pregnancy assessment, when the issues surrounding the original use of such an application are yet to be resolved. The use of a mid-pregnancy ultrasound assessment, where gestational age can be confirmed, and structural anomalies and markers of chromosomal aneuploidy can be recognised, is still debated. The assessment of newer imaging methods is easier than proving the value of older, well established ones.
The reports in the literature suggest that, given skilled staff and adequate equipment it is possible to confirm dates, identify most major structural anomalies, and through the use of morphological markers, achieve limited success in identifying pregnancies at increased risk of aneuploidy.
Unfortunately the combination of skilled personnel and modern equipment can be hard to come by, and ultrasonography remains one of the most operator dependent of diagnostic technologies. Any perceptions regarding the evolution of the mid-pregnancy scan must be considered in this context.
Throughout the nineties we have sought to find the true value of the mid-pregnancy scan, differentiating between the limitations of ultrasound and its users, constantly trying to provide the most accurate data on the capability of this investigation. Ultrasound manufacturers have taken advantage of revolutionary developments in computer technology to produce machines and transducers with even greater resolution, particularly at higher frequencies.
This evokes the question of the future of the mid pregnancy assessment, in terms of both its timing and what is to be achieved by the investigation (if a detailed first trimester scan has been performed).
The paucity of anomalies left after first trimester anomaly and nuchal translucency screening, has lead us to reconsider the term ‘anomaly scan’ at 20 weeks gestation. In addition, the detailed early scan annuls the significance of ‘soft markers’ of aneuploidy e.g. mild hydronephrosis at the mid pregnancy scan. It is also more difficult to justify its use for this purpose in terms of cost benefit analysis. A scan at this time however, is likely to be most useful in the prediction of complications such as pre-term delivery and utero-placental disorders that present later in the pregnancy.
Colour Doppler technology has greatly improved access to the use of Doppler ultrasound, without necessarily resulting in a corresponding improvement in the skill of the operator. This problem can only be addressed by proper training and meticulous attention to detail. While the reproducibility of uterine artery Doppler ultrasound screening in pregnancy varies, it is generally accepted that there is a strong relationship between elevated uterine artery resistance and perinatal complications, particularly pre-eclampsia. The finding of elevated resistance in both uterine arteries (bilateral notches) at 20 or 24 weeks gestation identifies a cohort of women at considerable risk of early delivery as a result of uteroplacental problems. The next logical step is to identify and introduce interventions that will prevent or palliate such complications; studies are currently being conducted to address the issue.
Finding suitable prophylactic therapies has proved more difficult than originally envisaged. There remains a nagging suspicion that aspirin, given in adequate amounts, can improve the outcome in certain high-risk pregnancies. Other regimens may also prove useful in this regard. For example, while glyceryl trinitrate, a nitric oxide donor, has a limited benefit as a single treatment, it may be of use in combination therapy. Meanwhile reports outlining the link between short cervical length and an increased risk of preterm delivery have been published. This opens a new debate as to whether screening for cervical length should be incorporated into the second trimester scan assessment.
Ultrasound in the management of high-risk pregnancies – more detailed information
Work undertaken in the nineties1, 2,3,4,9 have demonstrated the relationship between hypoxaemia/acidaemia and redistribution of the fetal arterial circulation. Abnormal venous waveforms (a reflection of cardiac dysfunction have been strongly linked to the development of acidaemia and impending heart failure, the final sequence seen in the fetus compromised by chronic utero-placental insufficiency (figure 1).5,6,7,8.
Umbilical arterial resistance typically informs us about placental resistance, whereas fetal growth, arterial and venous flow patterns provide an insight into the fetal response to inadequate placental supply of oxygen and other nutrients.
So where does this leave the fetal physician? How best is he or she to investigate and manage the at risk fetus? The first and most important point is to recognise that there is no “magic” score or figure that can be applied to all situations. Gestational age and fetal size are critical factors in any decision regarding delivery of the compromised fetus. Furthermore, Doppler indices are in some respects “gestational age conditional”: it is unusual to see abnormal venous flows at term, and the umbilical artery pulsatility index, nearly always elevated in preterm IUGR, can be normal in the term or post-term fetus that is compromised.
In our practice we use a combination of relevant medical and obstetric history, abnormal uterine-artery Doppler and referral on clinical suspicion of IUGR/pre-eclampsia as a signal to perform an initial assessment. If fetal growth, movement and amniotic fluid are normal, fetal Doppler or fetal heart rate analysis is not routinely performed. The decision regarding follow up is based on the findings at scan, in addition to the original indication for referral.
Where there is evidence of a reduction in growth velocity or evidence of growth restriction, where the amniotic fluid is reduced or no fetal movements recorded, a more detailed assessment of the fetus is undertaken. (All patients will typically have had an anomaly scan before 24 weeks gestation). If the umbilical and middle cerebral artery Doppler results are normal, we do not go on to perform venous flow or Doppler fetal heart rate assessment. Where there is evidence of compromise (cessation of growth, oligohydramnios, abnormal fetal arterial redistribution) a decision regarding delivery or further assessment is made, taking the gestational age, estimated fetal weight, biophysical and Doppler parameters into consideration.
When there is clear evidence of compromise e.g. cessation of growth, oligohydramnios, abnormal fetal arterial Doppler, the gestational age and estimated fetal size must be taken into account when making a decision regarding intervention. If the fetus is greater than 1500g and more than 34 weeks mature, it is difficult to find a good reason to continue with the pregnancy, particularly if the epidemiological data on size and subsequent complications holds true.
Between 32 and 34 weeks gestation other factors such as availability of neonatal and community services are additional important points when making a decision for continuing the pregnancy or arranging delivery.
Between 28 and 32 weeks gestation, with the fetus greater than 750g, the balance between gestational age and fetal compromise makes each decision unique. The risk of death or cerebral palsy reduces as each week goes by, but if delivery is delayed until there is fetal circulatory collapse (very abnormal venous blood flows), the risk of death is also increased. There are no easy answers in this situation. The patient, the neonatologist and the perinatologist have to weigh up each case and determine the optimum time for intervention on an individual basis. In these cases it would seem prudent to consider delivering the fetus, before venous blood flow patterns or short-term fetal heart rate variation become grossly abnormal, although we only have anecdotal clinical experience to support this approach.
With extreme prematurity, the decisions become even more difficult. The risk of death or handicap from immaturity is considerably greater in this group, when compared to later gestations. Fortunately, the small immature fetus often appears able to cope far better with hypoxia and acidaemia than its bigger and more mature equivalent. Here, the use of the computerised fetal heart rate analysis, Doppler the fetal arterial and venous circulation, in addition to growth and liquor, play a part in determining the frequency of assessments and the final decision to deliver.
Umbilical venous blood flow may be a useful method of assessing changes before there are frankly abnormal venous Doppler waveforms. On occasions a condition may be so severe as to consider not delivering at all, but allowing nature to takes its inevitable course. Of course on other occasions, the maternal condition e.g. pre-eclampsia precludes such a decision and requires the pregnancy to be ended as quickly as possible, in the mother’s interests.
In the absence of clear scientific data to support precise criteria for delivery, we must rely on clinical experience, interpreting all the available evidence to provide us with the best decision possible at the time. Where technology and the skill to utilise it is available, it would be irresponsible to base decisions on a limited range of information.
Further reading & information
- Soothill PW, Nicolaides KH, Campbell S. Prenatal asphyxia, hyperlacticaemia, hypoglycemia and erythroblastosis in growth retarded fetuses. BMJ 1987; 294: 1051-53.
- Bilardo C, Nicolaides KH, Campbell S. Doppler measurements of fetal and uteroplacental circulations: Relationship with umbilical venous blood gases measured at cordocentesis. Am J Obstet Gynecol 1990; 162: 115-20.
- Hecher K, Snijders R, Campbell S, Nicolaides K. Fetal venous, intracardiac and arterial blood flow measurements in intrauterine growth retardation: relationship with fetal blood gases. Am J Obstet Gynecol 1995; 173: 10-15.
- Rizzo, G., Capponi, A., Arduini, D., Romanini, C. (1996). Doppler indices from inferior vena cava and ductus venosus in predicting pH and oxygen tension in umbilical blood at cordocentesis in growth-retarded fetuses. Ultrasound Obstet Gynecol; 7: 401-10.
- Baschat AA, Gembruch U, Reiss I, Gortner L, Diedrich K. Demonstration of fetal coronary blood flow by Doppler ultrasound in relation to arterial and venous flow velocity waveforms and perinatal outcome-the “heart sparing effect”. Ultrasound Obstet Gynecol 1997; 8: 82-86.
- Harrington K, Thompson MO, Carpenter RG, Nguyen M, Campbell S. (1999) Doppler fetal circulation in pregnancies complicated by pre-eclampsia or delivery of a small for gestational age baby: 2. Longitudinal analysis. Br J Obstet Gynaecol, 106, 453-66.
- Kiserud T, Eik-Ness SH, Blaas H-G, Hellevik LR. (1994) Ductus venosus blood velocity and the umbilical circulation in the seriously growth retarded fetus. Ultrasound Obstet Gynecol; 4: 109-114.
- Gudmunsson S, Tulzer G, Huhta JC, Marsal K. (1996) Venous Doppler velocimetry in fetuses with absent end-diastolic blood velocity in the umbilical artery. Ultrasound Obstet Gynecol; 7: 262-7.
- Hecher K, Campbell S, Doyle P, Harrington K, Nicolaides K. Assessment of fetal compromise by Doppler ultrasound investigation of the fetal circulation. Arterial, intracardiac, and venous blood flow velocity studies. Circulation 1995; 91: 129-38.